[ty] fix lookup order of class variables before they are defined (#19743)

## Summary

This is a follow-up to #19321.

If we try to access a class variable before it is defined, the variable
is looked up in the global scope, rather than in any enclosing scopes.

Closes https://github.com/astral-sh/ty/issues/875.

## Test Plan

New tests in `narrow/conditionals/nested.md`.

crates/ty_python_semantic/resources/mdtest/narrow/conditionals/nested.md

@@ -346,7 +346,10 @@ l: list[str | Literal[1] | None] = [None]
 
 def f(x: str | Literal[1] | None):
     class C:
-        if x is not None:  # TODO: should be an unresolved-reference error
+        # If we try to access a variable in a class before it has been defined,
+        # the lookup will fall back to global.
+        # error: [unresolved-reference]
+        if x is not None:
             def _():
                 if x != 1:
                     reveal_type(x)  # revealed: str | None

crates/ty_python_semantic/resources/mdtest/scopes/global.md

@@ -262,3 +262,28 @@ def f():
     global __file__  # allowed, implicit global
     global int  # error: [unresolved-global] "Invalid global declaration of `int`: `int` has no declarations or bindings in the global scope"
 ```
+
+## References to variables before they are defined within a class scope are considered global
+
+If we try to access a variable in a class before it has been defined, the lookup will fall back to
+global.
+
+```py
+x: str = "a"
+
+def f(x: int, y: int):
+    class C:
+        reveal_type(x)  # revealed: int
+
+    class D:
+        x = None
+        reveal_type(x)  # revealed: None
+
+    class E:
+        reveal_type(x)  # revealed: str
+        x = None
+
+        # error: [unresolved-reference]
+        reveal_type(y)  # revealed: Unknown
+        y = None
+```